This invention relates to catalysts for oxidizing ammonia gas in contact with air or oxygen to produce nitrogen oxides, more particularly, to the composition of eutectic mixture catalysts comprising ferric oxides, bismuth oxide and other metal oxides. Therein, the ferric oxide and the bismuth oxide are the basic components of the composition.
Conventionally, single or multi-layers of nets of platinum or alloys of the platinum family metals, such as iridium and rhodium have been used as catalysts for oxidizing ammonia to produce nitrogen oxides. However, since the reaction temperature with the catalyst is from 800.degree. to 900.degree. C. and the life of the platinum nets is only 1/2 or 1 year, expensive platinum is rapidly used. Studies have been undertaken to substitute cheap nonmetallic catalyst for platinum. As a result a single-component catalyst, such as ferric oxide, cobalt oxide, manganese oxide, cupric oxide, vanadium (V) oxide, etc [Ind. Eng. Chem. 44, 1564 (1952), Ind. Eng. Chem. 46, 702-708 (1954), A.I. Chem. Eng. Journal. 11, 318-323 (1955), Chim. Ind. (Millan), 45, 15 (1963), Bul. Soc. Chim. 2042 (1972), J. Cat. 39, 57-72 (1975). U.S. Pat. No. 1952911] and multicomponent catalyst systems, such as ferric oxide-bismuth oxide, molybdenum oxide-bismuth oxide, ferric oxide-cobalt oxide, etc [Discussions Faraday Soc., 8140-8152 (1950), Chemie-Ing-Techn., 39 89-95 (1967), React. Kinet. Cat. Letters, 1, 4. 405-410 (1974), React. Kinet. Cat. Letters, 5,1, 55-60 (1976).], are reported for oxidizing ammonia to produce nitrogen oxides.
Among them, ferric oxide-bismuth oxide and ferric oxide-cobalt oxide have been known to be particularly useful. Although these multi-component catalysts show an oxidizing effect of above 95% in the temperature range of 600.degree. to 800.degree. C., their effective temperature range is very narrow and their mechanical strength is not suitable for industrialization.
Recently, in order to reinforce the mechanical strength of multi-component catalysts, the spinels, such as CaAl.sub.2 O.sub.4, etc., have been considered as carrier. However, the optimum ratio for components of the composition and stabilizer of nonmetallic catalyst have not been disclosed .